1. Field of the Invention
The present invention relates in general to systems for aligning the test head of an integrated circuit tester with an integrated circuit device under test (DUT) and in particular with a floating interface for a test head.
2. Description of Related Art
An integrated circuit (IC) tester tests an IC device such as a packaged IC or a die on an IC wafer by transmitting test signals to the device under test (DUT) and monitoring the output signal the DUT produces in response to the test signals. A IC tester includes a separate channel for each input/output (I/O) terminal or test point of the DUT, with each channel being capable of generating and transmitting a test signal to a corresponding DUT terminal or receiving and processing a DUT output signal produced at that terminal. The tester channels are typically implemented on circuit boards mounted in a relatively large chassis called a xe2x80x9ctest headxe2x80x9d. While a DUT usually has a large number of input/output (I/O) terminals or test points concentrated into a small area, the I/O ports of the channels within a test head are distributed over a much larger area. Therefore a test system must include some type of interface assembly capable of interconnecting the DUT and the test head I/O ports.
Typically a packaged IC to be tested is mounted on a printed circuit board called a xe2x80x9cloadxe2x80x9d board having a set of pads on its surface that are conductively linked by traces on the load board to the terminals of the IC. When die on a semiconductor wafer are to be tested, a xe2x80x9cprobe cardxe2x80x9d provides a set of probes to access test points on the die. A typical probe card will also have a set of pads on its surface that are linked to the probes through traces or other conductors. In either case, an interface assembly is provided to link the tester""s I/O ports to the pads on the surface of the load board or probe card.
An interface assembly, typically attached to the test head, includes a set of pogo pins or other resilient contactors to contact with the pads when the test head and load board or the probe card are brought together. Various types of conductors are used to link the tester I/O ports to the pogo pins.
To provide a good connection between all pogo pins and the pads on the probe card or load board, the pogo pin tips and the contacts must be properly aligned as they are brought into contact. Assuming the pads lie in some X,Y plane, the pogo pin tips must also reside in a plane parallel to the X,Y plan as they are brought into contact with the pads so that they will make contact with substantially equal force. Also the pogo pin tips must be in the proper rotational angle (xe2x80x9cthetaxe2x80x9d) about a (Z) axis perpendicular to the X,Y plane of the contact pads, and be properly positioned in X and Y directions within that plane in order to properly align with the pads.
Hence the test head must be rotated about the Z (xe2x80x9ctheta rotatedxe2x80x9d) to achieve the proper theta angle and rotated (xe2x80x9ctwistedxe2x80x9d) about some axis in the X,Y plane to position the plane of the probe tips parallel to the plane of the contacts. And finally the test head must be moved along the Z axis to a position where the probe tips come into contact with the load board or probe cards pads with proper force.
One problem with this approach is the test head can be quite massive, for example 500 pounds or more. Fine position adjustments of such a large mass are difficult to achieve. Since the massive test head has much inertia, relatively large forces must be applied to move it into position with reasonable speed, and the stresses involved with using large forces to move a massive object take their toll on parts.
Other structural factors also contribute to the difficulty of making proper alignment of the test head. A test head is usually mounted on a yoke permitting it to be twisted about only one axis in the X,Y plane. This limits our ability to adjust the plane of the pogo pin tips. Also the X axis about which the head can rotate passes through or near the center of gravity of the test head rather than through the contact pads, thereby making the movements needed for precise alignment complicated.
What is needed is a system for quickly and accurately aligning pogo pins or other contactors of an integrated circuit test head with pads or other contacts on a load board or probe card holding an integrated circuit device under test so that they make proper contact with one another.
A floating interface assembly in accordance with the invention provides signal paths between an integrated circuit (IC) test head and contact pads on a load board or probe card mounted on a platform adjacent to the test head. The load board or probe card includes conductors linking the contact pads to test points on the IC to be tested. Pogo pins or other contactors having tips for contacting the contact pads are mounted on the interface assembly and linked to the test head by flexible conductors.
In accordance with a first aspect of the invention, the interface assembly is attached to the test head by springs to allow it the freedom to move to some extent in any direction with respect to the test head and to rotate to some extent about any axis.
In accordance with a second aspect of the invention, as the test head approaches the contact pads, alignment pins mounted on the platform holding the load board or probe card engage bushings on interface assembly and orient the interface assembly so that its contactors will mate with the contact pads. As the contactors begin to contact the contact pads, the interface assembly adjusts the plane of the contactor tips so as to evenly distribute contact pressure over all contactors.
In accordance with a third aspect of the invention, the interface assembly includes a base plate connected by springs to the test head, a retainer plate for holding the contactors, and a hydraulic piston linking the base plate to the retaining plate. After the test head moves the interface assembly close to the contact pads, the hydraulic piston pushes the retainer plate away from the base plate so that the contactors make contact with the contact pads.
It is accordingly an object of the invention to provide an interface assembly for contacting contact pads linked to test points on an integrated circuit and for providing signal paths between those contact pads and test circuits within an integrated circuit tester.
The concluding portion of this specification particularly points out and distinctly claims the subject matter of the present invention. However those skilled in the art will best understand both the organization and method of operation of the invention, together with further advantages and objects thereof, by reading the remaining portions of the specification in view of the accompanying drawing(s) wherein like reference characters refer to like elements.